Update README.md

ppolischuk · web-flow · commit 54b14c211787 · 2021-09-01T13:46:31.000-07:00
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 # Crossref REST API
 
+# DEPRECATION NOTICE 
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+The Crossref REST API has been updated to use Elasticsearch instead of Solr -- read more about that project [here](https://www.crossref.org/blog/behind-the-scenes-improvements-to-the-rest-api/). 
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+This documentation is **deprecated**. Documentation for the new API can still be found at [https://api.crossref.org/](https://api.crossref.org/), and issues should be filed to the [Crossref issue repository](https://gitlab.com/crossref/issues) in Gitlab. 
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 - [Crossref REST API](#crossref-rest-api)
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 ## Preamble
 
 The Crossref REST API is one of [a variety of tools and APIs](https://www.crossref.org/services/metadata-delivery/) that allow anybody to search and reuse our members' metadata in sophisticated ways.
